Solid-State Lighting through Light Emitting Diodes (SSL-LEDs) involves the use of solid-state, inorganic semiconductor light emitting diodes to produce white light for illumination. Like inorganic semiconductor transistors, which displaced vacuum tubes for computation, SSL-LED is a disruptive technology that has the potential to displace vacuum or gas tubes used in traditional incandescent or fluorescent lighting. Advantages of SSL-LEDs over conventional light sources include: (1) higher efficiency and associated energy savings; (2) better color rendering; (3) small form factor; (4) ruggedness; (5) longer operational lifetime and low maintenance; (6) environmentally friendly; and (7) low fabrication costs.
Conventional LEDs typically generate monochromatic light with a narrow emission spectrum, and thus typically lack a broad emission spectrum to provide white light for illumination. In order to generate white light from an LED, a narrowband emission resulting from radiative recombination in the LED is transformed into broadband white light spectrum. Such broadband white light spectrum can be generated by three general approaches. A first approach is a wavelength-conversion approach by using an ultraviolet (“UV”) LED to excite multi-color phosphors that emit visible light at down-converted wavelengths. A second approach is a color-mixing approach by combining multiple LEDs, each of which generates light of a different color. A third approach is a hybrid between the two approaches described above. The current generation of commercially available white LEDs is primarily based on this hybrid approach. In particular, primary light emitted from a blue InGaN-based LED is mixed with a down-converted secondary light emitted from a pale-yellow YAG:Ce3+-based inorganic phosphor. The combination of partially transmitted blue and re-emitted yellow light gives the appearance of cool (green-blue) white light. Thus, phosphor coating technology is involved for white LEDs using either the wavelength-conversion approach or the hybrid approach.
Current approaches for phosphor coating are described next. A first approach, as depicted in FIG. 1A, is a slurry method involving the use of phosphor grains or particles 1 blended in a liquid polymer system, such as polypropylene, polycarbonate, epoxy resin, or silicone resin. The mixed phosphor slurry is dispensed on or surrounding an LED chip 2, and then the liquid polymer system is dried or cured. The LED chip 2 along with the phosphor slurry can be disposed in a reflector cup 3, as depicted in FIG. 1A. While the slurry method is a convenient phosphor dispensing method, a resulting color uniformity of LEDs manufactured with this slurry method is typically unsatisfactory, and colored rings can be observed from different viewing angles. These deficiencies are the result of: (1) variations in the thickness of a phosphor-containing material surrounding an LED chip can lead to various lengths of optical paths before an emitted light escapes a package; and (2) non-uniform phosphor distribution within the phosphor-containing material (because of gravity and buoyancy effects) tends to move larger phosphor particles downward during a liquid polymer curing process. Moreover, due to variations in the quantity of phosphor powders dispensed surrounding the LED chip, a white color coordinate tends to vary from device to device. These color variations, in turn, result in a complicated white LED color sorting process, the so-called color binning, which attempts to manage the color variations by sorting each device according to its white color coordinate.
To measure the uniformity of emitted light, the variation in a Correlated Color Temperature (“CCT”) can be used. A color temperature of a light emitting device can be determined by comparing its hue with a theoretical, heated blackbody radiator. A temperature, expressed in terms of degrees Kelvin, at which the heated blackbody radiator matches the hue of the light emitting device is that device's color temperature. An incandescent light source can be close to being a blackbody radiator, but many other light emitting devices do not emit radiation in the form of a blackbody curve and are, therefore, assigned a CCT. A CCT of a light emitting device is a color temperature of a blackbody radiator that most closely matches the device's perceived color. The higher the Kelvin rating, the “cooler” or more blue the light. The lower the Kelvin rating, the “warmer” or more yellow the light. By measuring the CCT at different light emission angles and comparing this variation among different light emitting devices, the uniformity of the light produced can be quantified. A blue LED chip dispensed with a yellow phosphor by the slurry method can have a typical CCT that varies from about 5,800 K to about 7,200 K across a range of 1,400 K for light emission angles at ±70° from a center light-emitting axis of the LED. Because of the presence of colored rings, the CCT is typically higher at or near the center axis than in the periphery, where the emitted light tends to be more yellow.
A second phosphor coating method is an Electrophoretic Deposition (“END”) method for the manufacture of phosphor-converted white LEDs, as depicted in FIG. 1B. In the case of END, a phosphor is electrically charged by adding a proper amount of an electrolyte in a liquid solvent to form a liquid suspension, and is biased by an electrical field. Then, surface charged phosphor particles are moved to an electrode of counter-polarity and coated on the electrode. EPD of the phosphor particles creates a phosphor layer 4 of relatively uniform thickness that can produce white light of greater uniformity and reduced instances of colored rings. While achieving greater color uniformity, the EPD method is generally lacking in its ability to deposit phosphors directly over an electrically nonconductive surface. In commercial production, a phosphor layer is typically coated directly over a LED chip 5, according to the so-called proximate phosphor configuration. This configuration tends to be inefficient in terms of light scattering, since the proximate phosphor layer can direct about 60% of total white light emission back towards the LED chip 5, where high loss can occur. Another drawback of the EPD method is that certain phosphors are susceptible to degradation by the solvent, thereby limiting the general applicability of the END method.
More recently and as depicted in FIG. 2, another approach involves forming a luminescent ceramic plate 6 by heating phosphor particles at high pressure until surfaces of the phosphor particles begin to soften and melt. The partially melted particles can stick together to form the ceramic plate 6 including a rigid agglomerate of the particles. The luminescent ceramic plate 6 is disposed in a path of light emitted by an LED chip 7, which is disposed over a set of electrodes 8. While providing benefits in terms of robustness, reduced sensitivity to temperature, and reduced color variations from chip to chip, a resulting package efficiency can be unsatisfactory due to the proximate phosphor configuration.
A scattering efficiency (also sometimes referred to as a package efficiency) is typically between 40% to 60% for commercially available white LEDs, with efficiency losses due to light absorption by internal package components such as an LED chip, a lead frame, or sub-mount. FIG. 3 depicts an example of a phosphor-converted white LED with yellow phosphor 31 powered by a blue LED chip 32, where a primary blue light 34 undergoing color mixing with a secondary light 35 of yellow color to generate a white color. A main source of light loss results from absorption of light by the LED chip 32. Because the LED chip 32 is typically formed of high-refractive index materials, photons tend to be trapped within the LED chip 32 due to Total Internal Reflection (“TIR”) once the photons strike and enter the LED chip 32. Another potential source of light loss results from imperfections in a mirror reflector 33 in the LED package.
Several scenarios depicted in FIG. 3 can direct light to the highly absorbent LED chip 32. First, a primary light 36 emitted by the LED chip 32 can be reflected back to the chip 32 by the phosphor powders 31 or by the mirror reflector 33. Second, down-converted secondary light 37 emitted by the phosphor powders 31 can scatter backward towards the LED chip 32. Third, both primary light and secondary light 38 can be reflected back towards the chip 32 due to TIR at an air-LED package interface. To improve the probability of light escaping from the package, a hemispheric lens 39 can be used to reduce instances of TIR at the air-package interface. To reduce instances of backward scattered light striking the LED chip 32, the phosphor powders 31 desirably should not be placed directly over the chip surface, but rather should be placed at a certain distance from the LED chip 32. Furthermore, a thinner phosphor layer would reduce instances of backward scattering of secondary light by the phosphor powders 31.
It is against this background that a need arose to develop the thin-film phosphor deposition process and related devices and systems described herein.